In 1824 Chief Justice John Marshall declared, in Gibbons v. Ogden, that “commerce” encompasses not merely “traffic”—“buying and selling, or the interchange of commodities”—but also all forms of commercial “intercourse,” including (in the case at hand) navigation. Moreover, such commerce may (indeed, must) extend into the interior of the states engaged in it, though it may not be “completely internal” to a state—i.e., neither “extend[ing] to” nor “affect[ing] other States.” In Cooley v. Board of Wardens of Port of Philadelphia (1851), the Supreme Court agreed with the state of Pennsylvania that it had the right, under an act of Congress in 1789, to regulate matters concerning pilots on its waterways, including the port of Philadelphia. The Court held that Congress had never intended to deprive the states of all power to regulate commerce. Specifically, where the commerce is not such as to require uniform regulation throughout the country and no relevant federal regulation exists, the states retain the power to regulate it until Congress, at a later date, enacts further legislation to restrict them.

That “selective exclusiveness” rule was affirmed and expanded upon in Southern Pacific Co. v. Arizona (1945), in which the Court found that

in the absence of conflicting legislation by Congress, there is a residuum of power in the state to make laws governing matters of local concern.

The Court in that case applied a three-part test to determine the implied condition to regulate interstate commerce: (1) that the law does not, in either its purpose or effect, discriminate against or excessively interfere with interstate commerce, (2) that the commerce in question is not such as to require national or uniform regulation, and (3) that the state’s interest in regulating such commerce is not outweighed by that of the federal government.

Although it is also generally held that the states may almost exclusively regulate intrastate commerce, Congress in fact does have the power to regulate such commerce in certain situations. In Swift & Co. v. United States (1905), for example, the Supreme Court held that a price-fixing scheme among Chicago meat-packers constituted a restraint of interstate commerce—and was therefore illegal under the federal Sherman Antitrust Act (1890)—because the local meatpacking industry was part of a larger “current of commerce among the States.” Congress’s power to regulate employers if their operations affected interstate commerce—even if they were not directly engaged in interstate commerce—was sustained in National Labor Relations Board v. Jones & Laughlin Steel Corp (1937). In the case of United States v. Darby (1941), although only some of the goods manufactured by Darby Lumber were to be shipped through interstate commerce, the Supreme Court held that the federal Fair Labor Standards Act (1938) could be applied to the intrastate production of those goods, because that production was part of the mainstream of the activity that would inevitably affect the interstate status of the goods.

In passing the Civil Rights Act of 1964, Congress relied on the commerce clause to prohibit racial segregation and discrimination in places of public accommodation involved in interstate commerce (Title II), among other provisions. In its unanimous (9–0) decision to uphold the law later that year (Heart of Atlanta Motel v. United States), the Supreme Court declared that

the power of Congress to promote interstate commerce also includes the power to regulate the local incidents thereof…which might have a substantial and harmful effect upon that commerce.

In 1995, for the first time in more than 50 years, the Court struck down a federal law as exceeding Congress’s regulatory authority under the commerce clause. In United States v. Lopez, the Court ruled that the Gun-Free Zones Act (1990), which prohibited the possession of a firearm within 1,000 feet of a school, was unconstitutional because the measure “neither regulates a commercial activity nor contains a requirement that the possession be connected in any way to interstate commerce.” In United States v. Morrison (2000), the Court held that the commerce clause did not permit Congress to enact a federal civil remedy—i.e., a ground for civil lawsuits in federal courts—for acts of gender-motivated violence as part of the Violence Against Women Act (1994). In 2005, however, the Court held in Gonzales v. Raich that enforcement of the federal Controlled Substances Act (1970) against the intrastate noncommercial possession, production, and use of medical cannabis (medical marijuana) in compliance with a California state law was consistent with the commerce clause because such activities could substantially affect the supply of and demand for marijuana in the illicit interstate market. The Court further limited the application of the commerce clause in the Affordable Care Act cases (2012), in which it largely upheld the Patient Protection and Affordable Care Act (PPACA) of 2010. Adopting a novel interpretation of the clause, the Court held that it applies only to commercial “activity,” not to commercial inactivity. Thus, the clause did not license Congress to include in the PPACA a provision that required individuals to purchase health insurance (the “individual mandate”), because the failure to purchase health insurance is not an activity in the ordinary sense. (The Court nevertheless upheld the individual mandate as a legitimate exercise of Congress’s taxing power.)